Systems and Methods for Recovering Desired Light Hydrocarbons from Refinery Waste Gas Using a Back-End Turboexpander

ABSTRACT

Systems and methods for recovering light hydrocarbons from refinery waste gas using a back-end turboexpander to generate a higher recovery of the light hydrocarbons for use as petrochemical feedstock and to remove the liquid light hydrocarbons before entering the turboexpander.

FIELD

The present disclosure generally relates to systems and methods forrecovering desired light hydrocarbons from refinery waste gas using aback-end turboexpander. More particularly, the present disclosurerelates to recovering desired light hydrocarbons from refinery waste gasusing a back-end turboexpander to generate a higher recovery of thelight hydrocarbons for use as petrochemical feedstock and to remove theheavier hydrocarbons before entering the turboexpander.

BACKGROUND

Gas from the streams in industrial applications, particularlyhydrocarbon refining operations, often include methane, otherconstituents, and light hydrocarbons having a molecular weight equal toor greater than ethylene, including ethylene, ethane, propylene,propane, butylenes and butane (hereinafter collectively referred to asthe “desired light hydrocarbons”). The desired light hydrocarbonstherefore, comprise. Recovery of the desired light hydrocarbons ispreferred because the desired light hydrocarbons are more valuable aspetrochemical feedstock than as refinery fuel gas. However, the systemsand methods for recovery of the desired light hydrocarbons are limited.

Current recovery of the desired light hydrocarbons in different refineryunits such as a saturated gas plant, a coker gas plant and a fluidcatalytic cracker (FCC) gas plant (collectively referred to as the“refinery gas plants”) is accomplished using absorption-stripping. Therecovery of propane using absorption-stripping is in the 90-94% rangewhile ethane is usually not recovered.

More recently, some of the desired light hydrocarbons have beenrecovered from the refinery waste gas using cryogenic systems. Thegeneral configuration of these cryogenic systems consists of firstcompressing, cooling and drying the feed gas to obtain a treated gas,followed by processing the treated gas through a turboexpander toproduce a two phase result. The lowest temperatures of these cryogenicsystems are reached in the turboexpander. The liquid generated from thetreated gas passing through the turboexpander is separated from thevapor and sent to a distillation column that separates the desired lighthydrocarbons from methane and other light components. The columnoverhead vapor and the turboexpander vapor are used as refinery fuelgas. Recovery of the ethane with this method is typically not more than80%.

Conventional cryogenic systems have shortcomings. The presence of heavyhydrocarbons in the treated gas that can result in undesirable freezingwithin the turboexpander, frustrating operation of the cryogenic system.Additionally, the efficiency of recovery of the desired lighthydrocarbons in convention systems is limited.

SUMMARY

The present disclosure overcomes one or more deficiencies in the priorart by providing systems and methods for recovering the desired lighthydrocarbons from refinery waste gas using a back-end turboexpander togenerate a higher recovery of the desired light hydrocarbons for use aspetrochemical feedstock and to reduce the extent of the heavyhydrocarbons entering the turboexpander.

In one embodiment, the present disclosure includes a system for recoveryof light hydrocarbons, which includes a gas chiller/dryer and adistillation column connected to the gas chiller/dryer for recoveringlight hydrocarbons.

In another embodiment, the present disclosure includes a method forrecovering light hydrocarbons from a gas stream, which includes treatingthe gas stream by compressing, amine-treating, drying and chilling toproduce a residue lighter gas, and separating the residue lighter gas ina distillation column between an overhead product and a raw columnbottom liquid product containing the light hydrocarbons.

Additional aspects, advantages and embodiments of the disclosure willbecome apparent to those skilled in the art from the followingdescription of the various embodiments and related drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is described below with references to theaccompanying drawings, in which like elements are referenced with likenumerals, wherein:

FIG. 1 is a schematic diagram illustrating a system for recovering lighthydrocarbons from refinery waste gas using a back-end turboexpander.

FIG. 2 is a schematic diagram illustrating another system for recoveringlight hydrocarbons from refinery waste gas using a back-endturboexpander and a heat exchanger.

DETAILED DESCRIPTION

The subject matter of the present disclosures is described withspecificity; however, the description itself is not intended to limitthe scope of the disclosure. The subject matter thus, might also beembodied in other ways, to include different steps or combinations ofsteps similar to the ones described herein, in conjunction with otherpresent or future technologies. Moreover, although the term “step” maybe used herein to describe different elements of methods employed, theterm should not be interpreted as implying any particular order among orbetween various steps herein disclosed unless otherwise expresslylimited by the description to a particular order. While the followingdescription refers refinery gas plants, the systems and methods of thepresent disclosure are not limited thereto and may be applied in otherrefineries to achieve similar results.

Referring now to FIG. 1, a schematic diagram illustrates a system 100for the recovery of desired light hydrocarbons from waste gas, inrefinery gas plants using a back-end turboexpander.

A raw feed gas, which contains light hydrocarbons, is obtained from acoker or FCC main fractionator overhead drum, or from any other source,such as refinery gas streams, gas streams used for fuel or to beeliminated as waste. The raw feed gas is provided to a gas chiller/dryer106 via a raw feed gas line 104. In the gas chiller/dryer 106, the rawfeed gas is compressed, is amine-treated to remove hydrogen sulfide andcarbon dioxide, if necessary, is dried and is chilled, to produce aresidue lighter gas. The residue lighter gas is then fed at an outflowthrough a residue lighter gas line 110 to a distillation column 112. Thedistillation column 112 includes a distillation column top section 112a, a distillation column bottom section 112 c, and a distillation columnintermediate section 112 d. The distillation column intermediate section112 d is intermediate the distillation column top section 112 a and thedistillation column bottom section 112 c. In the distillation column112, the desired light hydrocarbons are removed from the residue lightergas to produce a raw column bottom liquid product, and an overheadproduct containing methane, and lighter components.

A portion of the raw column bottom liquid product exits the distillationcolumn bottom section 112 c through a raw column bottom liquid productline 145 a. The system 100 also includes a reboiler 142, which drawsanother portion of the raw column bottom liquid product from a bottomsection 112 c of the distillation column 112 through a raw column bottomliquid product line 145 b. The reboiler 142 heats and recirculates rawcolumn bottom liquid product to the distillation column bottom section112 e.

The overhead product, a treated gas from which some of the desired lighthydrocarbons have been removed, leaves the distillation column 112 atthe distillation column top section 112 a through an overhead productline 116, and is sent first to a first liquid/gas separator 118, such asa knock out drum, to remove the small volume of liquid potentiallypresent in the overhead product, preventing that liquid from enteringthe back-end turboexpander 120. The back-end turbo-expander 120 wouldotherwise freeze any liquid, interfering with the operation of theback-end turboexpander 120. The remaining overhead product is thendelivered to the back-end turboexpander 120 where it is further cooledto produce a turboexpander two-phase product containing a condensedliquid, which acts as a reflux for the distillation column 112, and aremaining vapor. The turboexpander two-phase product is transmitted to asecond liquid/gas separator 126, such as a knock-out drum, through aturboexpander two-phase product line 124, where the turboexpandertwo-phase product is separated into the condensed liquid and theremaining vapor. The condensed liquid is sent through a condensed liquidline 132 to a pump 134 and then to the distillation column top section112 a as a reflux.

The remaining vapor is sent through a remaining vapor line 130 to theshell side of a shell and first tube condenser 112 b located in thedistillation column 112 near the distillation column top section 112 ato provide indirect cooling of column vapors within the distillationcolumn 112 and to exit as a residue gas. The first tube condenser 112 bincreases the efficiency of the distillation column in separating theconstituents of the residue gas. The first tube condenser 112 b mayconsist of spaced-apart vertical condenser tubes within a shell of thedistillation column 112 where the column internal vapors may flow insidethe condenser tubes. The first tube condenser 112 b is in communicationwith the remaining vapor line 130, so that the remaining vapor is fedinto the void between the vertical condenser tubes. The remaining vaporfrom the remaining vapor line 130 has a temperature lower than that ofthe column internal vapors within the distillation column 112, so thatthe remaining vapor acts as a cooling medium. While passing through thevoid between the vertical condenser tubes of the first tube condenser112 b, the remaining vapor absorbs heat from the column internal vaporswithin the distillation column 112, and exits as heated remaining vapor.Using the remaining vapor, which otherwise would be waste, as a coolingmedium in the first tube condenser 112 b, maximizes the limited value ofthe remaining vapor.

In a further embodiment, the distillation column 112 may be adeethanizer.

To further increase the efficiency of a deethanizer distillation column112, a second condenser 150 may be provided above the distillationcolumn intermediate section 112 d. The second condenser 150 permits theheat transfer from the heated contents of the distillation column 112,increasing the rate of distillation and therefore the efficiency of thedistillation column 112. A cooled refrigerant 152 is provided to thesecond condenser 150 and removed as heated refrigerant 154. The secondcondenser 150 may be external to the distillation column together withpiping and pumping to provide material from within the distillationcolumn 112 and back to the distillation column 112. Preferably, thesecond condenser 150 is internal to and within the distillation column112, so that external components can be minimized.

The second condenser 150 may consist of spaced-apart vertical tubeswithin a shell of the distillation column 112 where the column internalvapors may flow inside the condenser tubes. A cooled refrigerant 152,having a temperature lower than the residue gas within the distillationcolumn 112 is fed into the space between the condenser tubes within thedistillation column 112 to act as a cooling medium. While passingthrough the void between the vertical condenser tubes of the secondcondenser 150, the cooled refrigerant 152 absorbs heat from the columninternal vapors within the distillation column 112, and exits as heatedrefrigerant 154. The heated refrigerant 154 is then compressed,condensed, permitted to expand, and returned to the second condenser 150as cooler refrigerant 152.

Referring now to FIG. 2, a schematic diagram illustrates another system200 for the recovery of certain light hydrocarbons from waste gas usinga back-end turboexpander and a heat exchanger. The distillation column112 may be a demethanizer. Consistent with demethanizers, the system 200further includes a heat exchanger 202 in communication with thedistillation column 112 in the distillation column intermediate section112 d. The heat exchange 202 draws a portion of a partially-distilledliquid from the distillation column intermediate section 112 d, heatsthe portion of a partially-distilled liquid to produce a heated portionof a partially-distilled liquid, and provides the heated portion of apartially-distilled liquid to the distillation column 112 in thedistillation column intermediate section 112 d.

The system 100 may provide a higher recovery of the desired lighthydrocarbons, particularly propane and propylene, from the feed torefinery gas plants, as much as 99% compared to the 90-94% recovery inconventional absorber-stripper gas plants. Additionally up to 50% of theethylene and ethane can be recovered, if recovery of these components isdesired. The system 200 may provide an even higher recovery, in therange of 3-5%, of ethylene and ethane from the refinery waste gasescompared to the conventional configuration where the turboexpander ispositioned between the distillation column and the gas chiller/dryer.Each system also removes the liquid and heavy gas hydrocarbons beforeentering the turboexpander where they are likely to freeze. Each systemdisclosed may replace a conventional absorber-stripper design used inrefinery gas plants used to recover the desired light hydrocarbons. Eachsystem may also be retrofitted into existing refinery gas plants. Acryogenic gas plant utilizing either system would provide higherrecovery of propane and would permit recovery of part of the ethane inthe gas plant feed.

While the present disclosure has been described in connection withpresently preferred embodiments, it will be understood by those skilledin the art that it is not intended to limit the disclosure to thoseembodiments. For example, it is anticipated that by routing certainstreams differently or by adjusting operating parameters, differentoptimizations and efficiencies may be obtained, which would neverthelessnot cause the system to fall outside of the scope of the presentdisclosure. It is therefore, contemplated that various alternativeembodiments and modifications may be made to the disclosed embodimentswithout departing from the spirit and scope of the disclosure defined bythe appended claims and equivalents thereof.

1. A system for recovery of light hydrocarbons, which comprises: a gaschiller/dryer; a distillation column connected to the gas chiller/dryerfor recovering the light hydrocarbons; a first liquid/gas separatorconnected to the distillation column for separating the lighthydrocarbons in a liquid state from the light hydrocarbons in a gasstate; a turboexpander connected to the first liquid/gas separator forcooling the light hydrocarbons in a gas state to produce a condensedliquid and a remaining vapor; a second liquid/gas separator connected tothe turboexpander for separating the condensed liquid from the remainingvapor; a remaining vapor line connected to the second liquid/gasseparator for transporting the remaining vapor; a first condenserconnected to the remaining vapor line for cooling the distillationcolumn using the remaining vapor; a condensed liquid line connected tothe second liquid/gas separator for transporting the condensed liquid tothe distillation column for further processing; and a product line incommunication with the distillation column for removal of the lighthydrocarbons.
 2. (canceled)
 3. (canceled)
 4. The system of claim 1,wherein the distillation column is a deethanizer.
 5. The system of claim1, further comprising: a second condenser in the distillation column forcooling the distillation column using a cooled refrigerant.
 6. Thesystem of claim 1, wherein the distillation column is a demethanizer. 7.The system of claim 5, further comprising: a heat exchanger incommunication with the light hydrocarbons in the distillation column. 8.The system of claim 1, wherein the light hydrocarbons comprise ethyleneand the light hydrocarbons having a molecular weight greater thanethylene.
 9. A method for recovering light hydrocarbons from a gasstream, which comprises: treating the gas stream by compressing,amine-treating, drying and chilling to produce a residue lighter gas;separating the residue lighter gas in a distillation column between anoverhead product and a raw column bottom liquid product containing thelight hydrocarbons; removing a liquid from the overhead product prior toprocessing in a turboexpander; processing the overhead product throughthe turboexpander to obtain a condensed liquid and a remaining vapor;separating the condensed liquid and the remaining vapor before returningthe condensed liquid to the distillation column; returning the condensedliquid to the distillation column; and flowing the remaining vaporthrough a first condenser in the distillation column.
 10. (canceled) 11.The method of claim 9, further comprising: heating a portion of the rawcolumn bottom liquid product in a first heat exchanger to obtain aheated portion of the raw column bottom liquid product; and injectingthe heated portion of the raw column bottom liquid product into thedistillation column.
 12. The method of claim 9, wherein the distillationcolumn is a deethanizer.
 13. The method of claim 11, further comprising:providing a cooled refrigerant to a second condenser in the distillationcolumn.
 14. The method of claim 9, wherein the distillation column is ademethanizer.
 15. The method of claim 14, further comprising: heating aportion of the raw column bottom liquid product in a first heatexchanger to obtain a heated portion of the raw column bottom liquidproduct; and injecting the heated portion of the raw column bottomliquid product into the distillation column.
 16. The method of claim 15,further comprising: heating a portion of the raw column bottom liquidproduct in a second heat exchanger to obtain a second heated portion ofthe raw column bottom liquid product; and injecting the second heatedportion of the raw column bottom liquid product into the distillationcolumn.